Water-borne adhesives have experienced increased use relative to other materials such as solvent-borne and hot melt adhesives. More specifically, solvent-borne adhesives have become disfavored due to heightened environmental regulation. Hot melt adhesives are typically energy intensive and tend to possess relatively low heat resistance properties. Moreover, the performance characteristics of the solvent-borne and hot melt adhesives are often limited by the low molecular weight of the polymers employed.
Nonetheless, the end properties of water-borne adhesives are often adversely affected due primarily to the high water level of the adhesive polymer latexes. Specific disadvantages relate to grain raising on wood products, curling of paper products, and overall slow setting speed of the adhesive. Accordingly, recent efforts have focused on producing water-borne adhesives having increased solids content. This has been done in an attempt to enhance parameters relating to speed of adhesive set, water resistance, and adhesion to difficult-to-bond substrates. U.S. Pat. No. 5,434,216 to Mudge proposes vinyl acetate-based polymer emulsion formed of various monomers and having a solids content which may range between 40 and 50 weight percent.
U.S. Pat. No. 4,921,898 to Lenney et al. proposes a vinyl acetate-ethylene copolymer emulsion having a solids content between 65 and 70 weight percent and utilizing 2 to 4 weight percent of polyvinyl alcohol.
U.S. Pat. No. 5,070,134 to Oyamada et al. proposes a vinyl acetate ethylene copolymer emulsion having a solids content between 65 and 75 weight percent. Oyamada et al. employs a nonionic surface active agent having an HLB of from 16.5 to 17.5 per 100 parts by weight of the vinyl acetate unit.
U.S. Pat. No. 5,629,370 to Freidzon proposes a vinyl acetate ethylene copolymer emulsion having a solids content of at least 65 percent by weight. Freidzon employs a mixture of nonionic surfactants having an HLB of from 16.0 to 16.5.
In spite of any potential advantages, the above disclosed emulsions may be disadvantageous. The emulsions often display inferior rheological properties, and thus are may be difficult to process. Also, the emulsions may be unstable in that they tend to phase separate. As a result, the emulsions can present processing problems as well as possessing less than acceptable physical properties. Additionally, the emulsions may be raise potential environmental concerns.